Good friend has been flirting with my girlfriend for a whole week. She just told me yesterday, I don't know whether to forgive him or not.

Yesterday I was at the beach w/ my gf (lets call her kate) and she seemed a little different. With a little pushing, she finally told me she had been debating whether to tell me this all week or not. She was choked up, and she said my friend who knows her pretty well too had been facetiming her all week saying stuff about how he's always liked her and one day they'll be together and that he had a "plan" for them. Kate asked him if he realized she was dating his good friend, and he said he was just "playing around." She said it made her really uncomfortable and freaked out, and she didn't want to tell me because she thought I would be mad at her. I felt really betrayed and decided I was gonna call him that night, so I did and he denied everything sounding very guilty. He tried apologizing and acting funny like we were still good friends but I just ignored him. Now my friend Kat who is his best friend is telling me I should forgive him because we're such good friends and bros b4 hoes and all that shit. What do you guys think? I don't really see any reason why I should forgive him, he's done this kind of thing before to my female friend while she was dating this dude.